This invention generally relates to control systems for heating, ventilation and air conditioning (HVAC) systems. More particularly, embodiments of this invention relate to thermostats having jumperless designs and/or isolation circuitry.
In many single-stage heating and cooling systems, the heating system includes a low-voltage operated gas valve which controls the flow of gas to the furnace; the cooling system includes a contactor having a low-voltage coil and high-voltage contacts which control energizing of the compressor; and the circulation system includes a fan relay having a low-voltage coil and high-voltage contacts which control energizing of the fan which circulates the conditioned air.
The electrical power for energizing such low-voltage operated devices is provided either by a single transformer or by two separate transformers. If the heating and cooling system is installed as a complete unit, generally a single transformer is provided. Such a single transformer has the required volt-ampere output to operate all the low-voltage operated devices. If the cooling system is added to an existing heating system, sometimes an additional transformer is used.
For example, in a system originally designed to provide heating only, a fan relay is generally not provided since the fan is generally controlled directly by a thermal switch on the furnace. Therefore, in systems for providing heating only, the only electrical load on the transformer is often the gas valve. When the cooling system is subsequently added, the electrical load on the transformer increases due to the addition of the fan relay and the contactor. If the existing transformer does not have the sufficient volt-ampere output to operate all the low-voltage operated devices, an additional transformer is often added. Additionally, even if the additional transformer may not be necessary, it may nevertheless be installed so as to simplify the electrical wiring involved in the installation of the add-on cooling system.
It is desirable that a thermostat for controlling a single-stage heating and cooling system be constructed so as to enable it to be readily usable with either the single-transformer or two-transformer power source. A common approach is to electrically isolate the secondary windings of the two transformers from each other using a removable wire jumper. For example, see U.S. Pat. No. 4,049,973. U.S. Pat. No. 4,898,229 discusses a thermostat with integral means for detecting out-of-phase connection of a two-transformer power source, wherein an LED is used to indicate the out-of-phase connection to an installer. The installer is instructed to reverse the polarity of one of the two transformers if the LED is energized. U.S. Pat. No. 5,127,464 discusses a thermostat providing electrical isolation between connected heating and cooling transformers. However, the design nevertheless relies on a removable jumper to be manually inserted in the case where there is only a single HVAC transformer.
Thermostats in residential and light commercial buildings are often used to control multiple conditioning functions, such as heating, cooling, ventilating, etc. Often, a thermostat is designed such that the user must manually actuate a mechanical switch to change functions. For example, many thermostats have a mechanical switch with which the user can select from heating, cooling or fan functions. Some thermostat designs make use of mechanical relays for switching between functions such as heating, cooling, etc. Furthermore, many thermostat designs make use of relays for controlling each HVAC function, such that the relay within the thermostat is actuated each time the heating turns on or off, and each time the cooling turns on or off. However, it has been found that some users find the audible sound from actuating relays within the thermostat to be undesirable. Mechanical relays have a number of other disadvantages over solid state switching including larger size, reliability over time, and slower speed of switching.